As usual, Paul Graham has hit the nail on the head, this time regarding "stuff" - why we overvalue material possessions, collect too many of them, and let them exhaust us mentally:
...unless you're extremely organized, a house full of stuff can be very depressing. A cluttered room saps one's spirits. One reason, obviously, is that there's less room for people in a room full of stuff. But there's more going on than that. I think humans constantly scan their environment to build a mental model of what's around them. And the harder a scene is to parse, the less energy you have left for conscious thoughts. A cluttered room is literally exhausting.